1. Field of the Invention
The present invention is directed to a shockwave generator of the type preferably suitable for use in extracorporeal lithotripsy, and in particular to such a shockwave generator having an acoustic applicator contained therein for locating a calculus to be treated and for monitoring the progress of the treatment.
2. Description of the Prior Art
A shockwave generator having an ultrasound applicator therein, which is used to locate a calculus to be treated and to monitor the progress of the treatment, is disclosed in European Application 0 301 360, corresponding to U.S. Pat. No. 4,928,672. This shockwave generator permits non-invasive disintegration of calculi in a patient, and has a housing in which a shockwave source is contained. The shockwave source introduces shockwaves into a fluid propagation medium contained within the housing, and has an application end closed with a flexible sack. The shockwaves are focussed with an acoustic lens also disposed within the housing. To align the shockwave generator relative to the body of the patient to be treated, so that the calculus to be disintegrated is situated in the focus of the shockwaves, this known shockwave generator has an ultrasound applicator, which is part of an ultrasound locating system. The ultrasound applicator generates an ultrasound image of the region of the body which surrounds the focus. The ultrasound applicator is accommodated in a central bore of the shockwave generator so as to be longitudinally displaceable. The ultrasound applicator must be pressed against the body surface of the patient with the interposition of a flexible membrane. Although a region free of shockwaves is inherently present, as a consequence of the central bore which is provided for the acceptance of the ultrasound applicator, the applicator is nonetheless traversed when pressed against the patient's body by portions of propagating shockwaves, so that there is the risk that components inside of the ultrasound applicator will be damaged. Moreover, there is the risk that the housing surrounding the ultrasound transducer will be damaged due to cavitation effects so that water can penetrate into the oil-filled interior of the ultrasound applicator housing, resulting in failure of the ultrasound applicator.
German utility model 88 09 253 discloses a similar shockwave generator. In this structure, also, an acoustic lens is provided for focussing the shockwaves generated by the shockwave source. To correct for imaging errors, a hollow, conical deflection member is arranged in the propagation path of the shockwaves. The deflection member has a surface consisting of brass or steel. Water is provided as the propagation medium for the shockwaves. An ultrasound applicator of an ultrasound locating system is disposed in the interior of the hollow, conical member. The hollow, conical member is intended to protect the ultrasound applicator against the influence of the shockwaves. Because the fundamental frequency of the shockwaves generally lies at 0.5 MHz, and a noticeable acoustic blocking effect can only be achieved when the wall thickness of the hollow, conical member is at least the same as a wavelength of the fundamental oscillation of a shockwave propagating in the material of the hollow, conical member, a wall thickness of at least 9 mm would be required, given brass as the material of the hollow, conical member. This is based on a sound propagation speed of approximately 4500 m/s in brass. Such a wall thickness of the hollow, conical member is not practical, if only for space reasons. To protect the ultrasound applicator against parts of the shockwaves which penetrate through the wall of the hollow, conical member, the interior of the hollow, conical member is filled with an expanded plastic material, which absorbs the portions of the shockwaves which penetrate through the wall. An effective protection of the ultrasound applicator is possible in this known device, therefore, only if the hollow, conical member has a sufficient wall thickness, and also if a sufficiently thick layer of expanded plastic material is present between the wall and the ultrasound applicator. This known solution also requires a substantial structural space. Moreover, specific measures for holding the hollow conical member must be provided, further increasing the structural outlay.